Method of making integrated circuit embedded with non-volatile programmable memory having variable coupling

ABSTRACT

A programmable non-volatile device is made with a floating gate that functions as a FET gate that overlaps a portion of a source/drain region and allows for variable coupling through geometry and/or biasing conditions. This allows a programming voltage for the device to be imparted to the floating gate through variable capacitive coupling, thus changing the state of the device. Multi-state embodiments are also possible. The invention can be used in environments such as data encryption, reference trimming, manufacturing ID, security ID, and many other applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/271,666, filed Nov. 14, 2008, which is a continuation-in-part ofU.S. patent application Ser. No. 12/264,029, now U.S. Pat. No.7,782,668; Ser. No. 12/264,060, now U.S. Pat. No. 7,787,304; and Ser.No. 12/264,076, now U.S. Pat. No. 7,787,309, all filed Nov. 3, 2008,which claim priority of U.S. Provisional Patent Application No.60/987,869, filed Nov. 14, 2007, all of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to making non-volatile memories withvariable coupling which can be programmed one time, or multiple times insome instances. The invention has particular applicability toapplications where is it desirable to customize electronic circuits.

BACKGROUND

One time programmable (OTP) and multi-time programmable (MTP) memorieshave been recently introduced for beneficial use in a number ofapplications where customization is required for both digital and analogdesigns. These applications include data encryption, reference trimming,manufacturing ID, security ID, and many other applications.Incorporating OTP and MTP memories nonetheless typically comes at theexpense of some additional processing steps.

A new form of OTP is disclosed in the aforementioned U.S. applicationSer. No. 12/264,029 and which is incorporated by reference herein. Inthat disclosure, a new type of single-poly non-volatile memory devicestructure can be operated either as an OTP (one time programmable) or asan MTP (multiple time programmable) memory cell is disclosed. The devicestructure is fully compatible with advanced CMOS logic process, andwould require, at the worst case, very minimal additional steps toimplement. A unique aspect of the device is that the floating gate ofthe memory cell structure is electrically coupled strongly through oneof the S/D junctions of the transistor, whereas traditional single polynonvolatile memory cells require either an additional interconnect layerto couple to the floating gate, or the floating gate has virtually noneor minimal electrical coupling to any of the existing electricalsignals.

Another key feature is that it is implemented with an NMOS devicestructure, whereas the traditional single-poly OTP is commonlyimplemented with a PMOS device structure. This means that the device canbe formed at the same time as other n-channel devices on a wafer.

Another advantage of an NMOS device structure is that it behaves similarto an EPROM device, i.e., the device is programmed into a non-conductingstate from a conducting state. (The most commonly used PMOS OTP deviceis programmed from a non-conducting state into a conducting state). Thiscan eliminate the need of an additional masking step that is commonlyassociated with a PMOS OTP device in order to make sure that PMOS deviceis in a non-conducting state coming out of the manufacturing fab. Inaddition, since an NMOS device's programming mechanism with channel hotelectrons injection is self-limiting, unlike that case of a PMOS withchannel hot electron programming, the amount of energy consumptionduring programming is self-limited for this invention.

An additional benefit of the aforementioned device is the fact thatmulti-level functionality can be incorporated very easily by simplyemploying different forms of variable electrical coupling as discussedbelow. The ability to have OTP and MTP cells capable of storing nbits—instead of merely one—is believed to be unique to theaforementioned device.

Another NMOS OTP implementation is disclosed by U.S. Pat. No. 6,920,067,incorporated by reference herein. The device in this reference isprogrammed with channel hot-hole-injection. The disclosure teaches thatthe device is programmed into conducting state, after the channel hothole injection. However, it is unclear whether the device actually worksin the way the inventors claim. That is, it is not apparent that thechannel current will be initiated to induce hot-hole-injection since thestate of the floating gate is unknown and there is no available means tocouple a voltage unto the floating gate. An NMOS device will conduct achannel current to initiate the hot hole injection only when thefloating gate potential is sufficient to turn on the device, or when thethreshold voltage is always low initially to allow channel currentconduction. The only way to ensure either scenario is to introduce anadditional process step to modify the turn on characteristics of theNMOS. Now assuming the channel is conducting initially and hot holes areinjected, the holes injected on the floating gate will make the devicemore conductive. So the device basically goes from a conductive state(in order to initiate channel current for hot hole injection) to ahighly conductive state. This is not a very optimal behavior for amemory device.

Another prior art device described in U.S. publication no. 2008/0186772(incorporated by reference herein) shows a slightly different approachto the problem of providing a programming voltage to a floating gateembodiment of an OTP device. In this design, shown in FIG. 4, the drainborder length L1 is increased relative to the source side length L1 toincrease a coupling ratio to the eraseable floating gate 416. Byincreasing the coupling ratio, the amount of channel current isincreased; therefore the charge injection into the floating gate willalso increase. The drawbacks of this cell, however, include the factthat the cell and channel 412 must be asymmetric, and the coupling isonly controlled using the length dimension of the active regions.Because of these limitations, it also does not appear to be extendableto a multi-level architecture. Moreover, it apparently is onlyimplemented as a p-channel device.

Accordingly there is clearly a long-felt need for a floating gate typeprogrammable memory which is capable of addressing these deficiencies inthe prior art.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to overcome theaforementioned limitations of the prior art.

A first aspect of the invention therefore concerns a programmablemulti-state non-volatile device situated on a substrate comprising: afloating gate; wherein the floating gate is comprised of a material thatis also used as a gate for a transistor device also situated on thesubstrate and associated with a logic gate and/or a volatile memory; asource region; and a drain region; and an n-channel coupling the sourceregion and the drain region; wherein the drain region overlaps asufficient portion of the gate such that a programming voltage for thedevice applied to the drain can be imparted to the floating gate throughcapacitive coupling; further wherein the device is adapted so that morethan one bit of information can be stored by the programming voltage.

In this multi-state embodiment, the device is preferably adapted suchthat during a read operation only a portion of the drain region receivesa read voltage. That is, a portion or all of the drain region can bebiased during a program operation to vary an amount of informationstored in the device. In some instances the device can be read by a biasvoltage applied to the drain region which is adjusted with time todetermine a threshold voltage of the floating gate.

In other preferred embodiments the floating gate can be erased to allowthe device to re-programmed. Preferably the floating gate is eraseableby an erase voltage applied to the source region.

In some applications the device can be integrated as part of aprogrammable array embedded with separate logic circuits and/or memorycircuits in an integrated circuit. Such circuit may be one of thefollowing: a data encryption circuit; a reference trimming circuit; amanufacturing ID; a security ID, or any other circuit that requirescustomized non-volatile data.

In some embodiments the capacitive coupling can take place in a firsttrench situated in the substrate. These trenches may be part of anembedded DRAM array. The amount of coupling can be tailored as desiredbased on selective control of a gate-interconnect mask, a source/draindiffusion mask, or both.

Other configurations can include a second programmable device coupled ina paired latch arrangement such a datum and its compliment are stored inthe paired latch.

To program the device to a multi-level state, a variable programmingvoltage is preferably used. This allows for multiple bits of data to bewritten by the programming voltage.

Another aspect of the invention concerns a multi-level one-timeprogrammable (MOTP) device situated on a substrate comprising: afloating gate; wherein the floating gate is comprised of a material thatis also shared by an interconnect and/or another gate for a transistordevice also situated on the substrate and associated with a logic gateand/or a volatile memory; a source region; and a drain regionoverlapping a portion of the floating gate, and the drain regionincluding at least a first drain region and a second selectable drainregion; wherein a variable capacitive coupling between the drain regionand the floating gate can be effectuated by one or more selectionsignals applied to the first drain region and the second drain regionrespectively; wherein the variable capacitive coupling causes a variableamount of channel hot electrons from the first drain region and from thesecond drain region to permanently alter a threshold value of thefloating gate and store multi-bit data in the OTP device.

A further aspect of the invention concerns the fact that in someembodiments, the device has a multi-level (multi-bit) programmed statedefined by an amount of charge stored on the floating gate by thevariable programming voltage.

Other aspects of the invention concern methods of forming theaforementioned multi-level non-volatile programmable memory device.

Still other aspects of the invention concern methods of operating theaforementioned multi-level non-volatile programmable memory device. Inpreferred embodiments an amount of capacitive coupling can be adjustedbased on altering a number of N (N>1) separate drain regions selected tooverlap the floating gate and/or by altering a programming voltagelevel.

In instances where N=2, the threshold of the floating gate can be set toone of three (3) or four (4) different values as desired. When N=3 thethreshold of the floating gate can be set to one of eight (8) differentvalues, and so on. To read the state of the device, a read voltage ispreferably controlled to have a range of values which vary in timecorresponding to threshold states of the floating gate.

The multi-state device is preferably programmed with channel hotelectrons that alter a voltage threshold of a floating gate, and erasedwith band-band tunneling hot hole injection. In some embodiments thedevice is adapted so that different ones of the first drain region andthe second drain region can be coupled to the gate during program andread operations respectively. For example either, none or both the firstdrain region and the second region can be biased during a programoperation, and only one of the first region and the second region can bebiased during a read operation. Similarly, either or both of the firstregion and the second region can be biased during an erase operation.

Preferred embodiments of the multi-state device are n-channel, butp-channel can also be supported. In some applications the floating gatecan be implemented as a multi-level structure, as part of a thin filmtransistor, or even oriented in a non-planar configuration.

Another aspect of the invention concerns a single bit NV memory whichshares similar structural, formation and operating characteristics asthe multi-state device noted above.

Still another aspect concerns a one-time programmable (OTP) devicecomprising: a floating gate; wherein the floating gate is comprised of amaterial that is also shared by an interconnect and/or another gate fora transistor device also situated on the substrate and associated with alogic gate and/or a volatile memory; a source region; and a drain regionoverlapping a portion of the floating gate, and the drain regionincluding at least a first drain region and a second selectable drainregion; wherein a variable capacitive coupling between the drain regionand the floating gate can be effectuated by one or more selectionsignals applied to the first drain region and the second drain regionrespectively; wherein the variable capacitive coupling causes a variableamount of channel hot electrons from the first drain region and from thesecond drain region to permanently alter a threshold value of thefloating gate and store data in the OTP device.

The OTP device can be similarly configured structurally andoperationally as the multi-level device noted above. That is, an amountof capacitive coupling can be adjusted based on controlling/selecting anumber of N (N>1) separate drain regions, or the size of an overlap withthe floating gate, or using a variable programming voltage.

The devices are preferably embedded in a computing circuit and formedentirely by masks/CMOS processing used to form other logic and/or memoryn-channel devices in the processing circuit. In some instances thenon-volatile programmable memory device is used to store one or moreidentification codes for die/wafers.

It will be understood from the Detailed Description that the inventionscan be implemented in a multitude of different embodiments. Furthermore,it will be readily appreciated by skilled artisans that such differentembodiments will likely include only one or more of the aforementionedobjects of the present inventions. Thus, the absence of one or more ofsuch characteristics in any particular embodiment should not beconstrued as limiting the scope of the present inventions. Whiledescribed in the context of a non-volatile memory array, it will beapparent to those skilled in the art that the present teachings could beused in any number of applications.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top down view of a preferred embodiment of a non-volatilememory cell of the present invention;

FIG. 2 is a side cross section view of the preferred non-volatile memorycell;

FIG. 3 is an electrical diagram illustrating the electrical relationshipof the structures of the preferred non-volatile memory cell;

FIG. 4 depicts a prior art non-volatile memory cell which uses afloating gate for an OTP application;

FIG. 5 is an electrical diagram showing a preferred embodiment of alatch circuit constructed with the NV memory cells of the presentinvention.

FIG. 6A is a top down view of a preferred embodiment of a non-volatilememory cell of the present invention which uses variable coupling;

FIG. 6B is an electrical diagram illustrating the electricalrelationship of the structures of the preferred non-volatile memory cellusing variable coupling.

DETAILED DESCRIPTION

The present disclosure concerns a new type of non-volatile memory devicestructure (preferably single poly) that can be operated either as an OTP(one time programmable) or as an MTP (multiple time programmable) memorycell using variable capacitive coupling. The preferred device structureis fully compatible with advanced CMOS logic process, and would require,at the worst case, very minimal additional steps to implement.

A unique aspect of the present device is that the floating gate of thememory cell structure is electrically coupled strongly through avariable number of S/D junctions of the transistor, whereas traditionalsingle poly nonvolatile memory cells require either an additionalinterconnect layer to couple to the floating gate, or the floating gatehas virtually none or minimal electrical coupling to any of the existingelectrical signals. Moreover, unlike the 2008/0186772 reference, thecoupling ratio can be more specific and precise. That is, by exactlycontrolling the coupling ratio (through areal means) the amount ofcharge, and thus the final programmed Vt, are directly proportional tothe product of the coupling ratio and the drain voltage. It can be moreprecisely controlled such that the coupling ratio is dictated ordesigned by the desired programming threshold level (V.sub.t) of thememory cell. This allows for a design that evolves easily into amulti-level version of an OTP since different coupling ratios yielddifferent programmed V.sub.t.

FIG. 1 illustrates the top view of the layout of a preferred structureused in the present invention. FIG. 2 illustrates a representativecross-sectional view of the device structure. It will be understood thatthese drawings are not intended to be set out to scale, and some aspectsof the device have been omitted for clarity.

The device includes a typical NMOS transistor 100 which is modified sothat the gate (poly in a preferred embodiment) 110 of the device is notelectrically connected to a voltage source. A drain 120 of the device isbent around and is preferably joined by an N-type well 130 thattypically already exists in a conventional advanced CMOS process. As analternative, the N-Well 130 can be replaced with an n-type diffusionlayer introduced so as to be beneath the poly floating gate. Aconventional source region 125 is also utilized.

The floating gate poly 110 is extended beyond a typical transistorchannel region 135 and includes an overlap region 140 which overlaps anactive region extending from the drain junction. The active regionportion 141 that is surrounded by the N-Well region serves as aneffective capacitive coupling to the floating gate. Thus any voltageapplied to the drain junction will be effectively coupled onto thefloating gate.

As seen in the electrical diagram of FIG. 3, if the coupling ratio ofthe drain to the floating gate is sufficiently high—which is determinedby the ratio of the area of the gate channel region and the area of thePoly extension overlapping the drain extension region—the floating gatecan effectively acquire and have a high percentage of the value of thedrain voltage.

A key advantage of the preferred embodiment, as seen in FIGS. 1 and 2,is that it is formed from same layers conventionally used to make activen-channel devices in a CMOS process. The only difference is that thepoly (or metal as the case may be) gate layer is not interconnected withsuch other formed active devices or coupled to a gate signal. The otherimplants for the source/drain are also part of a CMOS conventionalprocess. Thus, in most applications the invention can be integratedwithout any additional processing costs, because the only alteration isto an existing mask for each relevant layer of the wafer beingprocessed.

One other optional variation of this device structure is to make thedrain-to-gate coupling capacitor area on the sidewall of a trench. Thiswill greatly reduce the area of the drain-to-gate coupling capacitor.This reduction in cell area may come at the expense of significantlyincrease the manufacturing process complexity. However, again, inapplications where the invention is integrated with certain types ofDRAM architectures (especially embedded types), it is possible toincorporate the conventional processing steps for such memories to avoidadditional processing costs. Other techniques for coupling a voltage tothe floating gate and achieving a desired coupling ratio will beapparent to those skilled in the art.

While the floating gate is shown as a single polysilicon layer, it willbe appreciated by skilled artisans that other materials could be used aswell. In some applications for example it may be possible to exploit theformation of other structures/devices which while part of other mainunderlying logic/memory structures, can be exploited for purposes ofmaking a floating gate of some kind. In this respect it should be notedthat floating gates can typically be formed of a number of differentmaterials, including through techniques in which impurities areimplanted/diffused into a dielectric/insulating layer.

Moreover while the preferred embodiment depicts the NVM cell as part ofa conventional lateral-planar FET structure on a substrate, it will beapparent to those skilled in the art that other geometries/architecturescan be used, including non-planar structures. Thus the invention couldbe used in SOI substrates, in thin film structures, at other levels ofthe device than the substrate, in multi-gate (FINFET type) orientations,and in vertical/non-planar configurations. In such latter instances thefloating gate would be embedded and oriented vertically with respect tothe substrate.

The preferred operation of device 100 will be described. Thenon-volatile device structure preferably has the physical features of aconventional I/O transistor implemented in an advanced CMOS logicprocess. At present, such I/O transistor is nominally operated at 3.3Vbut it will be understood that this value will change with successivegenerations of manufacturing.

This type of I/O transistor typically has a threshold voltage of 0.5V to0.7V, with a typical electrical gate oxide thickness of 70 A. With adrain coupling to floating gate ratio of 0.90, and a read drain voltageof 1.0V applied to the device, the floating gate will effectively becoupled with a voltage of about 0.90V. This is sufficient to turn on theun-programmed NMOS device 100, and a channel current can be detected bytypical means of sense circuitry to identify the state of the device. Itwill be understood to those skilled in the art that the particularcoupling ratio, read voltage, etc., will vary from application toapplication and can be configured based on desired device operatingcharacteristics.

The device is originally in an unprogrammed state, which in thepreferred embodiment is characterized by a low resistance couplingbetween the source and drain through channel region 135. This means thatthe channel region 135 can be substantially uniform and current flow isreliable. While the preferred embodiment is shown in the form of asymmetric cell/channel, it will be understood that the invention couldbe used in non-symmetric forms such as shown in the aforementioned20080186722 publication.

To program the device into a programmed state, the device must be shutoff by reducing carriers in the channel region, and increasing thethreshold voltage. To do this a drain voltage of 6.0V can be applied andthis will effectively couple a voltage of about 5.4V to the floatinggate. This bias condition will placed the device into a channel hotelectron injection regime. The electrons injected into the floating gateeffectively increase the threshold voltage of the device. When asubsequent read voltage of 1.0V is applied again on the drain, thedevice does not conduct current due to its high threshold voltage, andthis second state of the device is thus determined. As with the readcharacteristics, it will be understood to those skilled in the art thatthe particular coupling ratio, program voltage, etc., will vary fromapplication to application and can be configured based on desired deviceoperating characteristics.

The prior art referred to above is primarily a one time programmabledevice, since there is no disclosed mechanism for removing the charge onthe floating gate. In contrast, some embodiments of the presentinvention can be made to be capable of multiple-time-programming. To dothis, an erase operation can be introduced to remove or neutralize theelectrons that have been injected into the floating gate. The mechanismfor removing or neutralizing electrons is preferably through band-bandtunneling hot hole injection from the other non-coupling junction 125 ofthe device. The preferred bias condition would be as followed: thenon-coupling junction (source junction) is biased with 6V to cause thejunction to initiate band-band tunneling current. The band-bandtunneling current causes hot holes to be injected into the floating gateand neutralize the electrons that are stored on the floating gate. Thusit is (re)programmed from a non-conducting, or even a low conductingstate, into a conducting state. The device is then able to conductchannel current when a subsequent read voltage is applied to thecoupling junction during the read operation. It will be understood thatprogramming from a low conducting state to a conducting state may have alimited operating sense window.

As an additional optional operation, to facilitate erase operation andenhance band-band tunneling current, the coupling junction can besupplied with a negative voltage so that the floating gate is made morenegative to cause higher band-band tunneling current across the sourcejunction.

Thus the operating characteristics are preferably as follows:

-   TABLE-US-00001 OPERATION Drain Source Substrate Program 6.0 V 0 V 0    V Read 1.0 V 0 V 0 V Erase Float or −Vcc 6.0 V 0 V

In some embodiments, additional protection can be implemented to ensurethe OTP and MTP device have sufficient immunity against the loss ofcharge stored on the floating gate. To do this, the device can beconfigured into a paired latch 500—as shown in FIG. 5—where the data andits complement are stored into the latch, thus effectively doubling themargin in the stored data. As seen therein, a top device 510 couples anode 530 to a first voltage reference (Vcc) while a second bottom device520 couples the node to a second voltage reference (Vss). By placingcharge on the top device floating gate, the top device 510 is programmedinto a non-conductive state, thus ensuring that node 530 is pulled downby bottom device 520 to Vss, representing a first logical data value(0). Similarly, by placing charge on the bottom device floating gate,the bottom device 520 is programmed into a non-conductive state, thusensuring that node 530 is pulled up by top device 510 to Vcc,representing a second logical data value (1).

Another useful advantage of the present preferred embodiment is that itis implemented with an NMOS device structure, whereas most traditionalsingle-poly OTPs are commonly implemented with a PMOS device structure.This means that the device can be formed at the same time as otherre-channel devices on a wafer. Another advantage of an NMOS devicestructure in this invention is that it behaves similar to an EPROMdevice, i.e., the device is programmed into a non-conducting state froma conducting state. In contrast, the prior art 20080186722 typedevice—and other commonly used PMOS OTP devices—are programmed from anon-conducting state into a conducting state. This aspect of theinvention thus can eliminate the need of an additional masking step thatis commonly associated with a PMOS OTP device in order to make sure thatPMOS device is in a non-conducting state coming out of the manufacturingfab.

In addition, since an NMOS device's programming mechanism with channelhot electrons injection is self-limiting, unlike that case of a PMOSwith channel hot electron programming, the amount of energy consumptionduring programming is self-limited for this invention.

As seen in the present description therefore, the particularconfiguration of the floating gate is not critical. All that is requiredis that it be structurally and electrically configured to controlchannel conduction and also be capacitively coupled to an electricalsource of charge carriers. The particular geometry can be varied inaccordance with any desired layout or mask. In some instances it may bedesirable to implement the floating gate as a multi-level structure forexample. Moreover, since capacitive coupling is a function of thematerials used, the invention allows for significant flexibility as thecomposition of the floating gate can also be varied as desired toaccommodate and be integrated into a particular process. An array ofcells constructed in accordance with the present teachings could includedifferent shapes and sizes of floating gates so that cells havingthreshold cells could be created.

Variable Coupling

In other embodiments of the invention, the effective coupling ratio ofthe device 100 can be made different/varied between read, program and/orerase operations. That is, while not shown in FIGS. 1, 2, the drainregion 120 coupled to the floating gate could be partitioned into one ormore separate sub-regions. This is shown in detail in FIGS. 6A and 6B.Each sub-region 121, 122, etc. may be fabricated or controlled to have adifferent amount of overlap with the floating gate. By selectivelyapplying a different voltage for one or more of such sub-regions,differing types of performance can be achieved for read/program/eraseoperations. For example it may be desirable to have an ultra low power(but somewhat slower) program or erase operations. This can be achievedby making a coupling area for such first type of operation smaller thanthe nominal area used during a second type (read) operation.

While in FIGS. 6A and 6B the variable coupling geometry is done byaltering a drain diffusion size (in a diffusion mask) and keeping afloating gate size constant, it will be apparent to those skilled in theart that the same effective result could be achieved by keeping a draindiffusion constant and altering a floating gate size. For example thefloating gate region 122 could be reduced in size to achieve the sameresult. By adjusting floating gate sizes it then becomes possible toshare diffusion regions as well, so that an adjacent floating gate 122′(for another cell) could be coupled to drain region 120. Othercombinations of these techniques will be useable as well and can beselected based on design/performance requirements.

Notably, the variable coupling aspects of the present invention can beused for both PMOS and NMOS OTP. Different coupling ratio options couldalso be used to impart different voltages on the floating gate, whichhas the potentially for multi-state storage, i.e., multi-level for anOTP.

As an alternative embodiment the programming voltage could be adjustedinstead of course, so that for a given drain coupling, the programmingvoltage applied to a particular cell is adjusted to write a differentstate to the floating gate. Because the drain is coupled to the floatinggate the variable programming voltage should be imparted to the floatinggate. For example, a drain voltage could be adjusted to have 3, 4 ormore different levels. This effectuates a different form of variablecapacitive coupling that may be more complex from a write perspectivebut may be useful in some applications.

A multi-level OTP variant for an NMOS implementation must take intoaccount that NMOS is programmed to an off state, so a little off is verysimilar to very much off, unless one uses different level of floatinggate voltages through different applied drain voltages to sense thestate. In such circumstances, however, the different drain voltage couldundesirably degrade the read disturb immunity, so there is a potentialtrade off here.

Another option for multi-level capability is this: since different drainto gate capacitance options are used (via different overlap area) toselect a programming state, a read operation can be performed throughthe same single drain overlap in order to detect the multi-state of thecell. For example, with two different drain overlaps, a total of 2 bitscould be implemented. In such an implementation, Drain 1 can be set tohave a coupling ratio (or overlap) which is some multiple (in this casepreferably 2) of that of Drain 2.

As the table shows below, a program voltage which imparts Drain 1 with avoltage of 0 and Drain 2 with a voltage of 6 V would write a first statein the cell, based on a first amount of charge imparted to the gate. Ifinstead all drains are programmed the charge added would be higher, thuscorresponding to a second state, and so on.

A total of four (4) different couplings (0 (no drains), 1.times. (Drain2), 2.times. (Drain 1), 3.times. (both Drain 1 and Drain 2))corresponding to four different threshold voltages, and thus fourdifferent logic states is achievable with this simple arrangement.

The preferable method of reading the state of the cell applies a readvoltage on both Drain 1 and Drain 2 as seen in Table 2 below. The amountof cell current is then sensed, which current is inversely proportionalto the amount of charge on the floating gate. The charge on the floatinggate as noted above is a direct function of the amount of couplingapplied during the programming. Thus the state of 0, 1.times., 2.times.and 3.times. in the cell can be detected by its relationship to theamount of read current. In this embodiment the read drain voltage ispreferably selected to be on the order of 1 volt. This has the advantageof preventing any kind of read disturb or drain induced leakagecontribution.

As an alternative which allows less decoding during read operation, theread can always be done on the Drain node with the highest couplingratio, in our example, the 2.times. Drain. In such instances it may notbe possible to differentiate between all 4 different states, but thismay be a desirable trade off in some applications.

As also shown in Table 2, as a further alternative to read themulti-level cell state, any single one (or combination) of the drains isbiased with a varying voltage over time to determine the coupled chargecontribution from the collective overlaps. The drain is biased with anincreasing voltage (from 0 to some target voltage sufficient to triggerthe gate in the highest threshold state) with time until the thresholdvoltage is achieved or decoded within a certain time interval at aparticular voltage level to identify the state of the cell. Thus whileall or some drains are biased during a program operation, only a singledrain need be biased (although others can, as above) during a read todetermine the state of the cell. The particular range of read drainvoltages will be dependent on the particular cell architecture, desiredoperating characteristics, etc. and can be determined by routinetesting. Again in this embodiment may not be possible to differentiatebetween all 4 different states, but this may be a desirable trade off insome applications. Other examples for programming and reading the cellwill be apparent to those skilled in the art.

-   TABLE-US-00002 OPERATION Drain 1 Drain 2 Source Substrate Program 0    or 6 V 0 or 6 V 0 V 0 V Read 1 volt (or 0—N 1 volt (or 0 V 0 V volts    in M volt 0—N increments) volts in M volt increments) Erase Float or    −Vcc Float or −Vcc 6.0 V 0 V

Other ratios are possible, of course, subject to the restriction that byselecting different ratios which are not multiples of 2, the sensingmargin/differentiation may not be as great. However, in some instancesit may be desirable to set the respective overlaps to some higher/lowermultiple, which would have the effect of reducing a sense margin betweentwo adjacent states at some range of output. If there is an imbalance inthe sensitivity of the sense range, however, this may be a desirableoption (i.e., if it is easier to detect the difference between 1.times.and 2.times. than it is for 4.times. and 5.times. or vice versa).Furthermore in some cases it may be satisfactory to reduce the number ofeffective logic states by sacrificing one of the combinations to resultin an odd number of logic states. For example, a multi-bit cell may have3 programmed drain couplings simply of {0, Drain 1, Drain 2} thusignoring {Drain 1+Drain 2}.

While two separate coupling ratios are shown in FIG. 6A and threeseparate coupling ratios are shown in FIG. 6B, it will be understoodthat other partitionings and couplings could be implemented inaccordance with the present teachings. In the case of FIG. 6B, forexample, 8 different programmed states can be achieved by using 3different levels of charge coupling. For example, different combinationsof drains having coupling ratios of 1.times., 2.times. and 4.times. canbe combined, or some other set of ratios. Again, other selections couldbe made with fewer logic states in exchange for higher margin betweenstates. Other variations of the invention will be apparent to thoseskilled in the art.

The above descriptions are intended as merely illustrative embodimentsof the proposed inventions. It is understood that the protectionafforded the present invention also comprehends and extends toembodiments different from those above, but which fall within the scopeof the present claims.

What is claimed is:
 1. A method of forming a multi-level one-timeprogrammable (MOTP) memory cell incorporated on a silicon substrate withone or more other additional logic and/or non-MOTP memory devices,characterized in that: a. said MOTP memory cell has a drain regioncapacitively coupled to a floating gate; b. any and all regions andstructures of said MOTP memory cell are formed in common withcorresponding regions and structures used as components of theadditional logic and/or non-MOTP memory devices; and c. an amount ofcapacitive coupling between said floating gate and said drain region canbe varied during a program operation to store multiple bits of datawithin a single MOTP memory cell.